The present invention relates to a liquid crystal display device having bistable nematic liquid crystal and also to a method of driving the liquid crystal display device.
Liquid crystal displays are classified into two types. The first type is known as a transmissive display which uses the light emanating from a back light. The second type is known as a reflective display which utilizes external light such as natural light and light emitted from lamps on the ceiling. A liquid crystal display of either type comprises a front polarizing plate, a rear polarizing plate, and a liquid crystal cell interposed between the polarizing plates. A reflective liquid crystal display further comprises a reflective plate laid on the outer surface of the rear polarizing plate.
The liquid crystal cell comprises a pair of substrates and liquid crystal held in the gap between the substrates. Each of the substrates has a number of electrodes arranged on its inner surface and an aligning film covering the electrodes. The aligning film aligns the molecules of the liquid crystal near the substrate, in a specific aligned state (e.g., twist-aligned state).
The liquid crystal cell has a number of pixels. Each pixel comprises one electrode provided on the inner surface of the first substrate, one electrode provided on the inner surface of the second substrate, and that part of the liquid crystal which is located between these two electrodes. When a drive signal is applied between the electrodes of the pixel, the molecules of the liquid crystal are tilted to the substrates and allow the passage of light. How much light is passed through the liquid crystal depends on how much the liquid crystal molecules are tilted to the substrates.
Two types of liquid crystal cells are known. The first type is a simple matrix cell in which the electrodes provided on the inner surface of the first substrate intersect with those provided on the inner surface of the second substrate. The second type is an active matrix cell which is identical in structure to the simple matrix cell, except that one active element is connected to each electrode. The simple matrix cell is advantageous because it is more simple than the active matrix cell and can therefore made at a lower cost.
In a liquid crystal display having a simple matrix cell, the effective value of a drive signal applied between the electrodes of each pixel for a predetermined time is controlled to display an image. To display a gray-scale image, the drive signal is time-divided into segment drive signals. The segment drive signals are sequentially applied between the electrodes of the pixel, thereby changing stepwise the amount of light passing through the pixel. The more minutely the drive signal is time-divided, the smaller the difference between the effective values of any two segment signals which correspond to two adjacent gray-scale levels. In other words, when the liquid crystal cell is driven in high-duty time division, the operating voltage margin (i.e., the difference between the effective values of the segment signals) is inevitably too narrow for the display to provide clear gray-scale images.
The liquid crystal display having a simple matrix cell can hardly be driven in high-duty time division. Therefore, the display cannot have an increased number of pixels to display clear gray-scale images.